The present invention relates to a CDMA receiving apparatus in a CDMA communication system for transmitting a transmit data which are subjected to a spread modulation processing with a predetermined spreading factor and demodulating the transmit data by subjecting the received signal to a despread processing and, more particularly, to a CDMA receiving apparatus provided with a function of estimating the error rate in a plurality of energy power ratios Eb/N0s which are lower than the Eb/N0 in the current communication.
As a modulation system in mobile communication, digital systems are now mainly used in place of the conventionally used analog systems. Analog cellular systems are generally called a first generation, while digital cellular systems such as PDC (Japanese standard), GSM (European standard), IS54 (US TDMA standard) and IS95 (US CDMA standard) are called a second generation. The systems of the first and second generations are mainly used for voice service, and effectively utilize limited radio bands for communication by making the most use of the analog/digital narrow-band modulation demodulation system.
In a next generation, however, not only telephone communication but communication through a FAX and an electric mail, and communication of inter-computer etc. are possible. In a next generation, therefore, there is a demand for a communication system which enables service of various types of information (multimedia information) such as a motion picture and a still picture in addition to sound and information through the above-described communication means, and which has such a high quality that a user is not aware that communication is performed in a mobile network. A DS-CDMA (direct sequence code division multiple access) system attracts attention as a promising radio access system which will satisfy the above-described demand. The DS-CDMA system is a system for realizing spread spectrum by directly multiplying the signal which is to be spread its spectrum by a signal in a much wider band.
FIG. 12 shows the structure of a CDMA transmitter in a mobile station. In a pilot channel signals are subjected to a BPSK modulation processing by a first modulator 1a and thereafter spread and modulated with a spreading code for the pilot channel by a first spreader 1b. On the other hand, in a data channel, after signals are subjected to an appropriate coding processing such as CRC coding and convolutional coding by an encoder 1c, they are subjected to a BPSK modulation processing by a second modulator 1d, and then spread and modulated with a spreading code for the data channel by a second spreader 1e. A multiplexer 1f combines these signals spread by the first and second spreaders 1b and 1e, respectively, into a vector. The combined signals are then mapped in I channel and Q channel In-phase channel and Quadrature channel which are orthogonal to each other, subjected to frequency conversion and high-frequency amplification by a radio transmitter 1g, and transmitted from an antenna 1h. 
FIG. 13 shows the structure of a CDMA receiving unit for 1 channel in a CDMA receiver at a base station. A radio receiver 2a converts the frequency of the high-frequency signal received from an antenna into the frequency of baseband signals, and inputs the baseband signals into a searcher 2b and each of finger portions 2c1 to 2cn. When the direct sequence signal (DS signal) which is influenced by the multi paths are input, the searcher 2b detects the multi paths by autocorrelation using a matched filter (not shown), and inputs the timing data for starting despread and delayed time adjusting data in each path into each of the finger portions 2c1 to 2cn. A despreader 3a in the pilot channel of each of the finger portions 2c1 to 2cn subjects the direct wave or delayed wave which arrives through a predetermined path to a despread processing by using the same code as the spreading code in the pilot channel, integrates the result of the despread processing, thereafter subjects the integrated signal to a delay processing corresponding to its own path, and outputs a pilot channel signal. A despreader 3b in the data channel of each finger portion subjects the direct wave or delayed wave which arrives through a predetermined path to a despread processing by using the same code as the spreading code in the data channel, integrates the result of the despread processing, thereafter subjects the integrated signal to a delay processing corresponding to its own path, and outputs a data channel signal.
A channel estimating portion 3c estimates a channel for compensating for the influence of fading in a communication path by using the despread pilot channel signal, and outputs a channel estimation signal. A fading compensator 3d compensates for the fading of the despread data channel signal by using the channel estimation signal. A RAKE combiner 2d combines the signal output from each of the finger portions 2c1 to 2cn, and outputs the combined signals to a decoder 2e for soft decision error correction as a soft decision data train. The decoder 2e decodes and outputs the transmitted data by soft decision error correction, and inputs the decoded data into an encoder 2f. The encoder 2f subjects the decoded data with an error corrected to the same encoding processing as the encoder 1c (FIG. 12) in the transmitter, and a error rate estimator 2g estimates a bit error rate BER by comparing the result of encoding with the data before decoding. The bit error rate BER is usable for the control of transmission power.
FIG. 14 is an explanatory view of the transmission power control in a closed loop of an uplink. In a mobile station 1, a spread modulator 11 spreads and modulates a transmit data by using a spreading code which corresponds to a predetermined channel designated by a base station 2, and a power amplifier 12 amplifies the input signal which is subjected to a processing such as orthogonal modulation and frequency conversion after the spread modulation, and transmits the amplified signal from an antenna to the base station 2. In the base station 2, a despreader 21 which corresponds to each path subjects a delayed signal which arrives through an allocated path to a despead modulation processing, and a RAKE combiner/demodulator 22 combines the signal output from each finger.
An Eb/N0 measuring portion 23 measures the ratio Eb/N0 which is the ratio of the signal energy per bit Eb to the noise power N0 of a received signal. A comparator 24 compares the target Eb/N0 with the measured Eb/N0 and if the measured Eb/N0 is larger than the target Eb/N0, it creates a command for lowering the transmission power by TPC (Transmission Power Control) bits, while if the measured Eb/N0 is smaller than the target one, it creates a command for raising the transmission power by TPC bits. The target Eb/N0 is a value necessary for obtaining a bit error rate BER of, for example, 10xe2x88x923 (an error occurs once in 1000 times), and it is input into the comparator 24 from a target Eb/N0 setting portion 25. A spread modulator 26 spreads and modulates the transmit data and the TPC bits. After the spread modulation, the base station 2 subjects them to processing such as DA conversion, orthogonal modulation, frequency conversion and power amplification, and transmits them from the antenna to the mobile station 1. A despreader 13 in the mobile station 1 subjects the signal received from the base station 2 to a despread processing, and a RAKE combiner/demodulator 14 decodes the received data and the TPC bits and controls the transmission power of the power amplifier 12 in accordance with the command designated by the TPC bits. The mobile station 1 and the base station 2 perform the above-described power control in every slot.
In the above-described transmission power control, it is necessary to determine and set the target Eb/N0 in such a manner as to obtain a bit error rate BER of, for example, 10xe2x88x923. If a communication environment is bad, for example, it is necessary to set the target Eb/N0 at a large value in order to obtain a desired bit error rate BER. On the other hand, if the communication environment is good, it is possible to obtain a desired bit error rate BER even if the target Eb/N0 is small. It is therefore necessary to measure the present communication environment, and set the target Eb/N0 in such a manner as to obtain the desired bit error rate. In the prior art, however, it is unfavorably difficult to measure the communication environment and set the target Eb/N0 in such a manner as to obtain the desired bit error rate in a short time.
Furthermore, in the prior art, it is necessary to control the transmission power so that the measured Eb/N0 equals to the target Eb/N0 so as to obtain a desired bit error rate. In the prior art, however, the measured Eb/N0 is compared with the target Eb/N0 and the transmission power is controlled by predetermined quantity using TPC bits in accordance with the result of the comparison. For this reason, it inconveniently takes a long time to obtain the target Eb/N0, in other words, a desired bit error rate.
In addition, although it is necessary to report the bit error rate BER in the current Eb/N0 to a host apparatus in transmission power control, it is impossible to estimate the bit error rate BER in the current Eb/N0 in a short time.
Accordingly, it is an object of the present invention to eliminate the above-described problems in the related art, and to set a target Eb/N0 which achieves a desired bit error rate in a short time.
It is another object of the present invention to reduce a control time which is required in order to obtain a desired bit error rate in transmission power control.
It is still another object of the present invention to estimate a bit error rate in a predetermined Eb/N0 in a short time.
To achieve the above-described objects, in the present invention, there is provided a CDMA apparatus comprising: (1) a first despread signal generator for multiplying a received signal by the same code as a spreading code on the transmission side in every chip, dividing the results of the multiplications in all the chips into a plurality of groups, and summing the results of the multiplications in each group so as to output a plurality of despread signals having a smaller spreading factor than the spreading factor on the transmission side; (2) a second despread signal generator for generating despread signals of a predetermined spreading factor by summing every k despread signals out of the plurality of despread signals output from the first despread signal generator, and (3) an error rate estimator for estimating a bit error rate in each spreading factor by judging the transmit data from each of said despread signals in each spreading factor. If the spreading factor becomes xc2xd, the energy Eb per bit becomes xc2xd, so that the Eb/N0 decreases by 3 dB. If the spreading factor becomes xc2xc, the Eb/N0 decreases by 6 dB, if the spreading factor becomes xe2x85x9, the Eb/N0 decreases by 9 dB and the same rule applies correspondingly to the following. Accordingly, the error rate estimator is able to estimate the error rates in Eb/N0s which are lower than the Eb/N0 in the current communication on the basis of the error rate judged in each spreading factor.
To state this concretely, (1) when the first despread signal generator divides the results of the multiplications of the spreading code and received signals for all the chips into 2n groups, it sums the results of the multiplications in all the chips each group, and outputs a plurality of despread signals having a spreading factor of SF/2n, wherein SF is the spreading factor on the transmission side, (2) the second despread signal generator sums 2m (mxe2x89xa6n) despread signals out of the plurality of despread signals output from the first despread signal generator, and outputs despread signals having a spreading factor of S/2(nxe2x88x92m), wherein m is variable so that the second despread signal generator outputs despread signals having various spreading factor, and (3) the error rate estimator judges the transmit data in each bit from each despread signal and estimates the bit error rate in each spreading factor by using the result of the judgment.
In this manner, the error rate estimator is able to count the frequency of errors in each spreading factor (in other words, the frequency of errors in each Eb/N0) in one despread processing for decoding one bit data, and to estimate the bit error rate BER in each spreading factor by accumulating the frequencies of errors during a predetermined time. It is therefore possible to estimate the error rates in a plurality of Eb/N0s which are smaller than the Eb/N0 in the current communication in a short time, and to decide and set the target Eb/N0 which produces a desired bit error rate by interpolation using these plurality of error rates. In addition, since the target Eb/N0 which produces a desired bit error rate is confirmed in a short time, it is possible to obtain a desired bit error rate in a short time by immediately controlling the transmission power so that the actual Eb/N0 equals to the target Eb/N0.
Furthermore, it is possible to calculate the error rate in the Eb/N0 in the current communication by interpolation from the error rates in a plurality of Eb/N0s which are lower than the Eb/N0 in the current communication, and to report the error rate in the Eb/N0 in the current communication to a host apparatus in a short time.